I’ve made a very simple portable USB charger, using a 9V battery as a power supply. I’m using this to charge my MicroTrack when I’m in a middle of a field recording and power runs out.

I’ve used a 7805 voltage regulator IC, 2 pieces of 0.1uF electrolytic capacitors, a switch, battery holder, USB A connector, heat sink for the regulator, some wires and a plastic box. You can view the schematics here.